The present invention relates to a method for reducing the content of alkali metal in an organo-soluble cellulose ether.
Organosoluble cellulose ethers such as ethyl cellulose and benzyl cellulose are known to be particularly useful in a wide variety of applications including the preparation of molded articles, e.g., extruded films or tubes, and as a component in protective coatings, adhesives, lacquers and ink or binder compositions.
Conventionally, the organosoluble cellulose ethers are generally prepared by the reaction of an alkali cellulose (typically, the reaction product of a cellulose and an alkali metal hydroxide) with an etherifying agent, e.g., ethyl chloride or ethyl oxide, in the presence of a reaction diluent such as toluene. The resulting cellulose ether is conventionally recovered from the reaction medium by desolvating the reaction product, thereby forming solid particles of the cellulose ether, converting any alkali metal hydroxide to salt form and washing the cellulose ether particles several times with water to remove the soluble alkali metal salts and other impurities therefrom. The washed cellulose ether is then dewatered and dried. Unfortunately, to remove the necessary amount of the alkali metal salt by conventional methods requires large amounts of water, e.g., about 5 to 40 times the weight of the cellulose ether, from which water the cellulose ether must be recovered. As such, substantial expenditures of time, energy and apparatus are necessary to effectively produce a purified cellulose ether.
Heretofore, several methods for preparing a cellulose ether having a low alkali metal salt content have been proposed. For example, U.S. Pat. No. 2,744,894 teaches that purified hydroxyalkyl ethers of polysaccharides can be prepared by etherifying the cellulose in a proper reaction diluent, e.g., a solvent mixture of a benzenoid hydrocarbon and a monhydric alcohol, and extracting the alkali metal salts from the polysaccharide ethers prepared therein using a suitable rinse solvent such as a mixture of methanol and acetone. While the disclosed method eliminates the necessity of a water wash, it is undesirable for the reason that large amounts of the rinse solvent mixture are required to obtain a cellulose ether with the desired purity.
Alternatively, as disclosed in U.S. Pat. No. 3,347,847; the alkali metal salts are removed from a conventionally prepared hydroxyethyl cellulose by treating, i.e., cross-linking, the cellulose with glyoxal at an acid pH and subsequently washing the treated cellulose with water at a pH of from about 2 to about 6. Unfortunately, this method stills requires large amounts of wash water, e.g., about 2 to 50 times the weight of the cellulose ether. Improvements to the aforementioned method are disclosed in U.S. Pat. No. 3,903,076 which teaches that following etherification in a proper reaction diluent, the excess alkali metal hydroxide is neutralized and the water and water-soluble hydroxyl containing by-products removed therefrom by azeotropic distillation. The etherified cellulose is then cross-linked and washed with water. Although the azeotropic distillation effectively removes the hydroxyl containing ingredients, e.g., monoethers of glycol, the disclosed method does not substantially improve the prior art methods for reducing the alkali metal salt content in the cellulose ether.
Moreover, none of the disclosed methods are particularly useful in the preparation of purified organo-soluble cellulose ethers.
In view of the stated deficiencies of the prior art, it remains highly desirable to provide a method for effectively reducing the amount of alkali metal, in the form of a hydroxide or salt, from an organosoluble cellulose ether.